Sharepoint calculations in workflow

I'm using Sharepoint MOSS 2007 and Sharepoint Designer.

I have a list (form) that the user needs to fill out.  It has the standard name, date, category dropdowns, etc...  then there are 5 textboxes that the user can type info into, asking questions in those textboxes.  At the very bottom of the form, there is a textbox called "Total # of Questions".  This textbox needs to auto-populate with the number of the above Questions textboxes that were typed into.  Is this something that I would create a workflow for?  Or would it be done another way?  i tried creating a workflow, and created a variable, and had it populate to 1 if the textbox changed.  But it never seemed to trigger.  Not sure if I did it right.  Any help, suggestions, examples would be much appreciated.
Thanks!